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Stress Relaxation Testing, Creep Testing,
and Temperature & Load
Cycling
Creep is the slow dimensional change which will occur in a material
and hence a component when it is under constant load.
Stress relaxation is the loss in
stress with time in a component kept at constant deformation (strain)
- for example an o-ring seal.
Creep and stress relaxation rates can be measured at MERL to enable
correct material choice and correct design. Physical effects dominate
at low and ambient temperatures and these effects are largely reversible.
Chemical effects become increasingly important at higher temperatures
and are generally irreversible.
Under cyclic loads, the amount of creep may depend more on the number
of cycles than on the total time under load (known as fatigue-creep
interaction).
Under cyclic temperature conditions stress relaxation rates may be
significantly accelerated.
MERL can conduct various different forms of creep and stress relaxation
tests using its bespoke mechanical test machines with thermal cabinets,
or its pressure vessels for hydrostatic creep at high and low temperature.
MERL has developed a technique by which the sealing force in a seal
may be continuously monitored as the temperature around the seal changes. |
How the sealing force of a fluorocarbon seal in its
housing changes
during temperature cycles above and below ambient
temperature |
The figure above shows the behaviour of a fluorocarbon
O-ring seal in a standard housing. The seal is placed, and the initial
force
measurement made, at ambient temperature. Over a period of several
days the fixture is subjected to thermal cycling,
returning to room temperature between cycles.
The ambient temperature sealing force falls only slowly. However,
the
sealing
force would be
expected to rise and fall with temperature, but the rise in force seen
even at 80C, is well below what would be expected theoretically. This
is believed to be mainly due to the change in shape of the ring with
time (the 'set'). Quantitatively the results will depend on the geometry
of the seal and its housing. At present experimental studies such as
this, using the actual seal shape and housing design, are the only
method available for studying these phenomena. |
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